1. Field of the Invention
The present invention relates to a motor driving device, and particularly to a disk device such as a floppy disk drive device or a CD player device.
2. Description of the Prior Art
First, as an example of a conventional motor driving device, a floppy disk drive device will be described. FIG. 4 is a block diagram showing an outline of the configuration of a conventional floppy disk drive device.
As shown in this figure, the floppy disk drive device 10 (hereinafter referred to as the “FDD device 10”) has a head 30 for writing and reading signals to from a floppy disk 40 (hereinafter referred to as the “FD 40”) used as a magnetic recording medium.
When a signal is written to or read from the FD 40, the head 30 is moved stepwise in the direction of a radius of the FD 40 by a stepping motor 20 so as to be positioned at the target track at which the signal is to be written or read. On the other hand, the FD 40 is rotated at a constant linear speed by a spindle motor (not shown). In place of the stepping motor 20, a motor of any type may be used, such as a voice coil motor or a linear motor.
The operation of the stepping motor 20 is controlled by a stepping motor driver circuit 50 (hereinafter referred to as the “stepper circuit 50”). The stepper circuit 50 receives, from other blocks 60 including a control circuit and other circuits, step pulses that specify the number of steps (the amount of movement) over which the head 30 is to be moved stepwise, a step direction signal that specifies the direction in which the head 30 is to be moved stepwise, and other signals. On the basis of these signals, the stepper circuit 50 controls the stepping motor 20.
Moreover, the FDD device 10 shown in the figure is equipped with a USB (universal serial bus) as an interface bus, and the FDD device 10 is connected to a host computer (not shown) such as a personal computer via the USB. A USB is a standardized interface bus that has recently been receiving much attention as an interface bus for connecting a host computer with peripheral devices (a floppy disk drive device, a printer, and the like). A USB makes it possible to use a common interface among devices that have conventionally been demanding separate interfaces.
For easy connection and convenience, the USB includes power supply lines as well as signal lines. This permits a peripheral device equipped with a USB to be designed not to rely on separate power supply lines for the supply of electric power from outside (external power). In the present specification, a device that is designed to receive electric power from its host via a USB is called a “bus-powered” device; by contrast, a device that is designed to receive electric power not via a USB but via separate power supply lines from outside is called a “self-powered” device.
The FDD device 10 shown in the figure is a bus-powered device, and therefore it depends on the USB not only for the transfer of interface signals (hereinafter referred to as the “I/F signals”) between itself and the host computer but also for the supply of electric power from the host computer.
The I/F signals are exchanged directly between the host computer and the other blocks 60 provided in the FDD device 10. On the other hand, the electric power supplied from the host computer is fed not directly to the internal circuits of the FDD device 10, such as the stepper circuit 50 and the other blocks 60, but through a high-side switch circuit 70 to those circuits.
The high-side switch circuit 70 is a current limiter that limits the supply of current from the host computer to a predetermined level. Limiting the supply of current with the high-side switch circuit 70 serves to slow down the rising of the current when the FDD device 10 is plugged into the host computer, and thus helps suppress noise that accompanies the rush current.
In addition, at the output-side end of the high-side switch circuit 70, a bypass capacitor C1 is connected in parallel with the stepper circuit 50 and the other blocks 60. This permits noise components (alternating-current components) included in the output of the high-side switch circuit 70 to be diverted to ground. Thus, even if noise is superimposed on the electric power supplied to the FDD device 10, it rarely has an adverse effect on the internal circuits, such as the stepper circuit 50 and the other blocks 60.
As described above, the FDD device 10, designed as a bus-powered device, is very handy because it does not require separate power supply lines for the supply of electric power from outside. However, considering the burden on the host computer in terms of its power supply capacity, it is necessary to limit the electric power supplied to the FDD device 10. In fact, the standard regulating the USB sets a strict limit on the electric power that can be supplied via a USB, namely 5V, 500 mA at the maximum. Accordingly, the FDD device 10, designed as a bus-powered device, needs to be designed to consume as less electric power as possible, and therefore, as the stepping motor 20, a high-efficiency motor is used, which is expensive but requires a low driving current.
For example, driving a low-efficiency stepping motor such as is used in a self-powered FDD device (e.g. an FDD device built in a desk-top computer) requires a current as high as 200 to 300 mA. By contrast, driving the high-efficiency stepping motor 20 used in the bus-powered FDD device 10 requires a current as low as 80 to 90 mA.
The high-efficiency stepping motor 20, however, is supposed to be used in a comparatively narrow range of current. That is, a fluctuation in the current ISB that flows through the stepping motor 20 may cause improper operation thereof. The current ISB fluctuates with the supply voltage applied to the stepper circuit 50, and therefore, to increase the accuracy of the current ISB, it is necessary to reduce the fluctuation of the supply voltage. To achieve this, the conventional bus-powered FDD device 10 has a regulator circuit 80 provided as a stage preceding the stepper circuit 50.
FIG. 5 is a graph showing the relationship between the supply voltage VCC supplied to the FDD device 10 and the current ISB that flows through the stepping motor 20. In this graph, the supply voltage VCC is taken along the horizontal axis, and the current ISB is taken along the vertical axis. In the graph, the solid line L5 indicates the behavior of the current ISB; for comparison, the broken line L6 indicates the behavior of the current ISS that flows through a low-efficiency stepping motor in a self-powered FDD device.
As the solid line L5 indicates, thanks to the operation of the regulator circuit 80, the current ISB that flows through the stepping motor 20 has a range in which it remains constant irrespective of the supply voltage VCC. Thus, by using this range as the acceptable range of the supply voltage VCC, it is possible to drive the stepping motor 20 stably. Here, the current ISB is determined by the configurations of the regulator circuit 80 and of the stepper circuit 50, and is given byISB=(Vrag−Vsat)/Rmon  (1)
In formula (1) above, Vsat represents the stepper terminal saturation voltage generated within the stepper circuit 50, and Rmon represents the serial resistor of the stepping motor 20.
In the FDD device 10 configured as described above, it is possible to apply a predetermined voltage Vreg to the stepper circuit 50, and therefore it is possible to keep the current ISB that flows through the stepping motor 20 highly accurate. This helps stabilize the operation of the stepping motor 20. Moreover, at the output-side end of the regulator circuit 80, a bypass capacitor C2 is provided in parallel with the stepper circuit 50. This permits noise components (alternating-current components) included in the output of the regulator circuit 80 to be diverted to ground.
However, the FDD device 10 configured as described above requires that the regulator circuit 80 be externally fitted separately as a stage preceding the stepper circuit 50. This leads to a higher cost, and also to a larger area on the circuit board used. Moreover, for the stepper circuit 50 to be used as a common component irrespective of whether the stepping motor 20 actually used is of a high-efficiency or low-efficiency type, the regulator circuit 80 needs to be mounted and dismounted according to the specifications of the stepping motor 20. This makes the manufacturing process very ineffective.